US 7620504 B2 Abstract A driving-torque difference value, an inertial-force difference value of the vehicle, and an inertial-force change-amount difference value of the vehicle are calculated. Subsequently, a first determination coefficient by which the inertial-force difference value is to be multiplied or a second determination coefficient by which the driving-torque difference value is to be multiplied is estimated on the basis of a state equation having the inertial-force difference value as a state variable and the driving-torque difference value as an input variable. Subsequently, a road-surface condition is determined on the basis of a comparison between a threshold value and the first determination coefficient or the second determination coefficient.
Claims(4) 1. A road-surface condition estimating device, comprising:
driving-torque detecting means that detects a driving torque of an engine;
longitudinal-acceleration detecting means that detects a longitudinal acceleration of the vehicle;
driving-torque difference-value calculating means that calculates a difference between a currently detected driving torque of the engine and a previously detected driving torque of the engine as a driving-torque difference value;
inertial-force difference-value calculating means that calculates a difference between a current inertial force of the vehicle and a previous inertial force of the vehicle as an inertial-force difference value on the basis of the longitudinal acceleration;
inertial-force change-amount difference-value calculating means that calculates a difference between a current amount of change in inertial force of the vehicle and a previous amount of change in inertial force of the vehicle as an inertial-force change-amount difference value on the basis of the longitudinal acceleration;
determination-coefficient calculating means that calculates at least one of a first determination coefficient and a second determination coefficient on the basis of a state equation related to the inertial-force change-amount difference value, the state equation being formed by adding a first arithmetic term to a second arithmetic term, the first arithmetic term being obtained by multiplying the inertial-force difference value by the first determination coefficient, the second arithmetic term being obtained by multiplying the driving-torque difference value by the second determination coefficient; and
road-surface-condition determining means that determines a road-surface condition on the basis of the at least one of the first determination coefficient and the second determination coefficient.
2. The road-surface condition estimating device according to
3. A road-surface condition estimating device, comprising:
driving-torque detecting means that detects a driving torque of an engine;
longitudinal-acceleration detecting means that detects a longitudinal acceleration of the vehicle;
driving-torque difference-value calculating means that calculates a difference between a currently detected driving torque of the engine and a previously detected driving torque of the engine as a driving-torque difference value;
inertial-force difference-value calculating means that calculates a difference between a current inertial force of the vehicle and a previous inertial force of the vehicle as an inertial-force difference value on the basis of the longitudinal acceleration;
determination-coefficient calculating means that calculates at least one of a third determination coefficient and a fourth determination coefficient on the basis of a pulse transfer function related to the inertial-force difference value, the pulse transfer function being formed by adding a first arithmetic term to a second arithmetic term, the first arithmetic term being obtained by multiplying the inertial-force difference value by the third determination coefficient, the second arithmetic term being obtained by multiplying the driving-torque difference value by the fourth determination coefficient; and
road-surface-condition determining means that determines a road-surface condition on the basis of the at least one of the third determination coefficient and the fourth determination coefficient.
4. The road-surface condition estimating device according to
Description The disclosure of Japanese Patent Application No. 2007-166807 filed on Jun. 25, 2007 including the specification, drawings, and abstract is incorporated herein by reference in its entirety. 1. Field of the Invention The present invention relates to a road-surface condition estimating device in a vehicle that estimates a road-surface condition with high accuracy on the basis of a relationship between a road-surface friction coefficient and a slip rate of the wheels. 2. Description of the Related Art In recent years, there have been proposed and put in practical use various control technologies for vehicles, such as traction control, braking-force control, and torque-distribution control technologies. In many of these technologies, the calculation or correction of required control parameters is implemented in accordance with the road surface on which the vehicle is running and the grip condition of the tires. For example, Japanese Unexamined Patent Application Publication No. 6-323171 discloses a technology for setting a correction torque which is to be subtracted from a driving torque on the basis of a slip amount. To describe this technology in more detail, when the vehicle speed of a four-wheel drive vehicle is below a predetermined value of, for example, 20 km/h, a longitudinal acceleration detected by a longitudinal acceleration sensor is subjected to a filtering process. Subsequently, a filtered longitudinal acceleration having undergone a peak-hold process is selected. In contrast, when the vehicle speed is above or equal to the predetermined value, a longitudinal acceleration not having undergone a peak-hold process is selected. Then, the vehicle speed is determined by integrating the selected longitudinal accelerations. A difference between the determined vehicle speed and an average speed of a plurality of speeds included in the rotation speeds of the wheels is regarded as a slip amount, and a correction torque to be subtracted from a driving torque is set on the basis of this slip amount. In the technology disclosed in Japanese Unexamined Patent Application Publication No. 6-323171, however, the vehicle speed is determined by simply integrating the longitudinal accelerations but is not determined in view of the case where the road on which the vehicle is running is an ascending/descending slope. Therefore, the vehicle speed cannot be determined with high accuracy, which implies that the correction torque also cannot be determined with high accuracy. Specifically, referring to In this case, g indicates a gravitational acceleration, and θ indicates a slope angle of the road. A vehicle speed VB obtained by temporally integrating the longitudinal acceleration signal based on the equation (1) is as follows:
In this case, V Consequently, if the vehicle speed VB is estimated based on the detection value Gx detected by the longitudinal acceleration sensor when the vehicle is on an ascending slope, the estimated speed will unfavorably be higher than the actual speed by the amount of the gravity component. Similarly, the vehicle speed VB will be lower than the actual speed when the vehicle is on a descending slope, thus eliminating the ability to perform a proper slip detection. In view of the circumstances described above, it is an object of the present invention to provide a road-surface condition estimating device in a vehicle that allows for an estimation of a road-surface condition with high accuracy even when the road surface on which the vehicle is running is an ascending/descending slope. The present invention provides a road-surface condition estimating device in a vehicle, the road-surface condition estimating device including driving-torque detecting means that detects a driving torque of an engine; longitudinal-acceleration detecting means that detects a longitudinal acceleration of the vehicle; driving-torque difference-value calculating means that calculates a difference between a currently detected driving torque of the engine and a previously detected driving torque of the engine as a driving-torque difference value; inertial-force difference-value calculating means that calculates a difference between a current inertial force of the vehicle and a previous inertial force of the vehicle as an inertial-force difference value on the basis of the longitudinal acceleration; inertial-force change-amount difference-value calculating means that calculates a difference between a current amount of change in inertial force of the vehicle and a previous amount of change in inertial force of the vehicle as an inertial-force change-amount difference value on the basis of the longitudinal acceleration; determination-coefficient calculating means that calculates at least one of a first determination coefficient and a second determination coefficient on the basis of a state equation related to the inertial-force change-amount difference value, the state equation being formed by adding a first arithmetic term to a second arithmetic term, the first arithmetic term being obtained by multiplying the inertial-force difference value by the first determination coefficient, the second arithmetic term being obtained by multiplying the driving-torque difference value by the second determination coefficient; and road-surface-condition determining means that determines a road-surface condition on the basis of the at least one of the first determination coefficient and the second determination coefficient. The road-surface condition estimating device according to the present invention allows for an estimation of a road-surface condition with high accuracy even when the road surface on which the vehicle is running is an ascending/descending slope. Embodiments of the present invention will now be described with reference to the drawings. Referring to Based on these input signals, the control unit The driving-torque calculating portion In this case, i The calculated driving torque T is output to the driving-torque difference-value calculating portion The driving-torque difference-value calculating portion The inertial-force difference-value calculating portion Since an angle of an ascending/descending slope changes much more moderately as compared to slipping of the tires, the term corresponding to the gravity in the longitudinal acceleration signal can be considered as a constant between neighboring sampling time periods. Consequently, although an actual longitudinal-acceleration difference value ΔGx(n) can be obtained from the following equation (6) using the aforementioned equation (1), an inertial-force difference value ΔAx of the vehicle is calculated from the aforementioned equation (5) and the effect of a sloped angle θ is removed, considering that there is substantially no change in the value of the sloped angle θ (i.e. θ(n)=θ(n−1)).
The inertial-force change-amount difference-value calculating portion
Alternatively, the derivative value (dGx/dt) of the longitudinal acceleration Gx may be obtained from a signal from, for example, an additionally provided jerk sensor. The determination-coefficient calculating portion The state equation (8) mentioned above will now be described. Referring to The following equation (10) is a difference equation obtained on the basis of the equation (9).
In the course of the derivation of the equation (10), the term m·g·sin(θ) representing a gravitational component is considered as being fixed as in the description of the aforementioned equation (5). A total driving force Fd corresponding to tire characteristics is expressed with the following equation (11) based on a function μ of a slip rate λ (such as the one shown in
The reason that cos(θ)≈1 in the equation (11) is that a sloped angle of an actual road is 30% at the highest, and a cos function in that state is approximately 0.96. The following equation (12) is a difference equation obtained on the basis of the equation (11).
A slip rate λ is defined as λ=(ω−ω
By substituting the equation (13) into the equation (12), the following equation (14) can be obtained:
In this case, the wheel speed ωv in the equation (13) is a value converted to a wheel rotation speed by dividing the vehicle speed V by the tire radius R, but normally, a wheel speed ωv cannot be measured in a four-wheel drive vehicle. In the present invention, an inertial force of the vehicle, namely, a difference value of the vehicle acceleration Ax, can be determined from the longitudinal acceleration Gx in the aforementioned equation (5). Specifically,
Thus,
By substituting the equation (10) and the equation (16) into the equation (14), the following equation (11) can be obtained:
By applying the following equations (18) and (19) with respect to the equation (17), the aforementioned state equation (8) can be obtained.
The values A and B in the aforementioned state equation (8) can be estimated in real time using a so-called parameter identification method. For example, in the case where a recursive least square method (RLS method) is used, the following equations (20) to (22) are applied:
In this case, the aforementioned state equation (8) can be expressed with the following equation (23):
Using the following recurrence equation (24) with respect to the equation (23), an estimation value φe of a coefficient φ is determined.
In this case, f indicates a so-called decay function, and F(k) is determined from the following equation (25).
The first determination coefficient A and the second determination coefficient B in the aforementioned equation (8) are both constants including (dμ/dλ) as shown in the equation (18) and the equation (19). As shown in For example, in the case where (dμ/dλ) is a (dμ/dλ) value corresponding to a high μ road and the tires are generating a driving force, it can be considered that a sufficient grip force is maintained. If the value of (dμ/dλ) is close to zero, the tires are under a gross slip condition. In that case, it can be determined that there is a need to actuate some kind of slip suppressing means. If (dμ/dλ) is determined to be close to a (dμ/dλ) value corresponding to a low μ road, it can be estimated that the vehicle is running on a slippery road surface. In that case, various slip preventing devices may be set to a high standby mode so that these slip preventing devices can be actuated immediately when a slippage occurs. The road-surface-condition determining portion In the case where the first determination coefficient A is used, the determination is implemented as follows: When |A|≧K When K When |A|<K In this case, K In the case where the second determination coefficient B is used, the determination is implemented as follows: When |B|≦K When K When |B|<K In this case, K In the first embodiment, the determination of a road-surface friction coefficient μ is implemented based on three stages by comparing the preset threshold values K Instead of being constants, the threshold values K By setting the determination threshold values in accordance with the adjustment based on the wheel speed ω in order to determine the road-surface condition in the above-described manner, the estimation of a road-surface condition can be implemented with even higher accuracy. The road-surface condition (road-surface friction coefficient μ) determined at the road-surface-condition determining portion The road-surface estimating program executed by the control unit First, in step S In step S In step S In step S In step S In step S In step S According to the first embodiment of the present invention, a driving-torque difference value ΔT, an inertial-force difference value ΔAx of the vehicle, and an inertial-force change-amount difference value Δ(dAx/dt) of the vehicle are calculated, a first determination coefficient A by which ΔAx is to be multiplied or a second determination coefficient B by which ΔT is to be multiplied is estimated on the basis of a state equation having ΔAx as a state variable and ΔT as an input variable, and a road-surface condition is determined on the basis of the first determination coefficient A or the second determination coefficient B. This allows for an estimation of a road-surface condition not only at the grip limit of the tires but also over a wide running range. Even when the vehicle is running on a sloped road surface, the road-surface condition can be estimated with high accuracy without including errors caused by the slope. A second embodiment of the present invention will now be described. Referring to Based on these input signals, the control unit The determination-coefficient calculating portion As a characteristic of a pulse transfer function, it is known that the first determination coefficient A and the second determination coefficient B in the aforementioned state equation (8) and the determination coefficients P and Q in the pulse transfer function (30) have the relationships as shown in the following equation (31) and equation (32).
In this case, τ indicates a sampling time. Accordingly, each of the determination coefficients P and Q includes (dμ/dλ) that indicates the grip condition of the tires. As the tires approach a slip condition, the determination coefficient P approaches 1 and the determination coefficient Q approaches 0, whereby a road-surface condition can be detected. Since parameter identification methods such as an RLS method and a fixed trace method are widely known as methods for estimating the determination coefficients P and Q, the determination coefficients P and Q can be estimated using these methods. For example, in the case where the RLS method is used, the following equations (33) and (34) are applied:
Thus, the coefficient φ in the aforementioned equation (23) can be estimated from the equation (24). The road-surface-condition determining portion In the case where the third determination coefficient P is used, the determination is implemented as follows: When P≧K When K When P<K In this case, K In the case where the fourth determination coefficient Q is used, the determination is implemented as follows: When Q≧K When K When Q<K In this case, K In the second embodiment, the determination of a road-surface friction coefficient μ is implemented based on three stages by comparing the preset threshold values K Instead of being constants, the threshold values K By setting the determination threshold values in accordance with the adjustment based on the wheel speed ω in order to determine the road-surface condition in the above-described manner, the estimation of a road-surface condition can be implemented with even higher accuracy. The road-surface condition (road-surface friction coefficient μ) determined at the road-surface-condition determining portion The road-surface estimating program executed by the control unit First, in step S In step S In step S In step S In step S In step S According to the second embodiment of the present invention, a driving-torque difference value ΔT and an inertial-force difference value ΔAx are calculated, a third determination coefficient P by which ΔAx is to be multiplied or a fourth determination coefficient Q by which ΔT is to be multiplied is estimated on the basis of a pulse transfer function, and a road-surface condition is determined on the basis of the third determination coefficient P or the fourth determination coefficient Q. Similar to the first embodiment, this allows for an estimation of a road-surface condition not only at the grip limit of the tires but also over a wide running range. Even when the vehicle is running on a sloped road surface, the road-surface condition can be estimated with high accuracy without including errors caused by the slope. Since the road-surface condition is estimated on the basis of a pulse transfer function in the second embodiment, it is not necessary to determine a time derivative of the longitudinal acceleration. Normally, it is difficult to directly measure a time derivative of the longitudinal acceleration. Although a time derivative of the longitudinal acceleration can be calculated by differentiating data with respect to time-series detection values of a longitudinal acceleration sensor, since the longitudinal acceleration changes drastically, it is necessary to filter the signal from the longitudinal acceleration sensor in order to obtain a reliable derivative result. This filtering process unavoidably produces an adverse effect on the responsiveness. Japanese Unexamined Patent Application Publication No. 2006-023287 discloses a principle of a sensor for directly measuring a jerk, which is a time derivative of a longitudinal acceleration, and it may be possible to obtain highly reliable data using such a sensor. However, an addition of such a designated sensor can be problematic in terms of, for example, an increase in cost. In contrast, the second embodiment of the present invention can allow for an estimation of a road-surface condition with high accuracy without the need for such an additional sensor. Although the first and second embodiments described above are particularly directed to a four-wheel drive vehicle in which an estimation of a road-surface condition can be difficult, the first and second embodiments can also be applied to a two-wheel drive vehicle of a front-wheel drive type or a rear-wheel drive type. In that case, the longitudinal acceleration Gx is estimated with high accuracy based on the rotation speed information about the driven wheels that do not transmit a driving force to the road surface, whereby Ax and (dAx/dt) can be obtained without requiring a longitudinal acceleration sensor. In addition, if the vehicle load shared by the driving wheels is represented by md, the aforementioned equation (17), for example, can be expressed as the following equation (39):
Specifically, by changing the form of other equations in view of the proportion of the vehicle load shared by the driving wheels, the road-surface condition can be estimated under the same principle as described above on the basis of determination coefficients obtained from a state equation (first embodiment) or determination coefficients obtained from a pulse transfer function (second embodiment). Patent Citations
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